Search for quasi-particle scattering in the quark-gluon plasma with jet splittings in pp and Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

The ALICE Collaboration reports measurements of the large relative transverse momentum ($k{_T}$) component of jet substructure in pp and Pb-Pb collisions at center-of-mass energy per nucleon pair $\sqrt{s_{NN}}$=5.02 TeV. Enhancement in the yield of such large-($k{_T}$) emissions in head-on Pb-Pb collisions is predicted to arise from partonic scattering with quasiparticles of the quark-gluon plasma. The analysis utilizes charged-particle jets reconstructed by the anti-($k{_T}$) algorithm with resolution parameter R=0.2 in the transverse-momentum interval 60 < $p_{T},_{ch},_{jet}$ < 80 GeV/c. The soft drop and dynamical grooming algorithms are used to identify high transverse momentum splittings in the jet shower. Comparison of measurements in Pb-Pb and pp collisions shows medium-induced narrowing, corresponding to yield suppression of high-($k{_T}$) splittings, in contrast to the expectation of yield enhancement due to quasiparticle scattering. The measurements are compared to theoretical model calculations incorporating jet modification due to jet-medium interactions (“jet quenching”), both with and without quasiparticle scattering effects. These measurements provide new insight into the underlying mechanisms and theoretical modeling of jet quenching.